1. Field of the invention
The invention concerns a high-yield active printed-circuit antenna system for frequency-hopping space radar.
2. Description of the prior art
A sideways-looking SAR (Synthetic Aperture Radar) observation satellite can process echoes using the Doppler effect to obtain very fine image resolution in spite of the distance between the radar and the observed area but requires a very large antenna with two-dimensional electronic frequency agility, transmitting and receiving consecutively with two orthogonal linear polarizations.
A typical X band mission requires more than 6,000 phase control points on a 2.times.8 m antenna.
A centralized amplification system would have prohibitive losses and reduced reliability.
Space SAR antennas have already been implemented using two types of radiating element:
The European Space Agency ERS1 satellite antenna uses C band slot guides and a single V polarization ("pseudovertical" polarization perpendicular to the normal to the antenna), as described in the paper "The planar array antennas for ERS1" in "Proceedings of IGARSS 1988". Its beam is fixed, however, with no electronic scanning.
The antennas of the US Seasat, SIRA and B satellites are "patch" antennas made up of conductive areas ("patches") etched onto honeycomb material, resonating in the L band with a single H polarization (horizontal polarization, perpendicular to the normal to the antenna), as described in the paper "Seasat and SIRA microstrip antennas" in "Proceedings of Workshop on Printed Antenna Technology"--Las Cruces--1979". Their beam is fixed, however.
The SAR type SIR.C (Shuttle Imaging Radar) on the US space shuttle due to fly in 1991 or 1992 (see "Heading for space C. Band phased array" in "Microwave and RF" of April 1986) comprises:
an X band passive antenna using single polarization slot guides, PA0 two L and C band dual polarization active antennas.
Thus there is no X band active antenna or dual polarized antenna.
The prior art L and C band active antennas are scanned electronically in one plane only (elevation).
Additionally, the mass, thermal control and reliability constraints are less severe on the US shuttle; the transmit amplifiers use hybrid rather than monolithic technology, for example. The former technology involves a significant mass penalty.
Space SAR radars designed in France for terrestrial resource (vegetation, hydrology, oceanography) observation in particular must be able to operate in the X band (frequencies between 9.5 and 9.8 GHz) with both horizontal and vertical polarization.
The required radiating surface areas (height 2 to 3 m by length 7 to 10 m) rule out the juxtaposition on a satellite of different antennas, one for each polarization or one for transmission and the other for reception.
This French radar is a pulsed radar, successively transmitting a horizontal (H) polarization pulse, receiving echoes from a previous H pulse, transmitting a vertical (V) polarization pulse, and receiving echoes from a previous V pulse, as shown in the timing diagram (FIG. 1).
A duplicated switching system enables use of a single antenna scanned electronically in two planes and the whole surface of which is active for each of the previously mentioned four modes:
the printed-circuit radiating elements radiate (or receive) an electric field polarized either horizontally (H) or vertically (V) according to which of the two microstrip lines exciting the patches the port is switched to,
switching between the transmit (high-power) amplifiers and the receive (low-noise) amplifiers enables use of the same phase shifters to scan and to form the antenna beam.
If a centralized amplifier is used as in most of the SAR radars previously described, the losses in the distribution circuits between the satellite platform and the radiating elements, in the phase shifters and in the duplicated switching system are prohibitive:
they degrade the receive noise factor;
the transmission of pulses requires 3 to 6 kW of power, according to the mission: the above losses mean that the power available at the output of the TWT (travelling wave tube) must be approximately doubled. There is not currently available any X band pulsed TWT for space applications rated at 6 to 12 kW; even if such a device were to be developed, its reliability would be limited.